Laser systems such as applicants' assignee's laser systems, e.g., 7XXX and XLA-1XX, XLA-2XX and XLA-3XX model laser systems, with the 7XXX, being single chamber laser systems, e.g., ArF or KrF excimer laser systems, and the XLA models being multi-chamber laser systems, e.g., master oscillator-power amplifier (“MOPA”) configured laser systems, e.g., excimer MOPAs, may be used for such applications as above noted, e.g., laser annealing of materials on a workpiece substrate and as integrated circuit photolithography DUV light sources. The latter of which systems currently operating at 6 kHz and can produce about 60-90 watts delivered to, e.g., a photolithography tool in a burst of hundreds of pulses with about 10-15 mJ per pulse. This operation is relatively close to maximum due, e.g., to high pulse energy and concomitant optical damage problems. Also limitations exist such as in the form of fan size and speed needed to circulate the gas in the lasing chamber(s).
In order to preserve the endless march of Moore's law and with practical and economical EUV light source production perhaps not arriving quite on time, the photolithography tool makers have turned to a technology known as immersion lithography, whereby a fluid is interposed between the wafer and the mask/retical and projection lens assembly.
While much of the photolithography tool may still be utilized for immersion techniques, the DUV 193 nm light source will need to be significantly changed, in order to meet higher throughput requirements, and also for beam stability reasons (bandwidth, dose, center wavelength, etc.).
A further motivation for implementing a very high average power, e.g., 100 W, laser system, e.g., as an XLA laser system or other power amplification stage, is that further advances in lithographic resolution can be achieved through a “double exposure” technique. Scanner makers will no doubt want to produce a stepper that can provide double exposure with no loss in wafer throughput. Thus, wafer stage speed (scan speed) would also have to increase by approximately 2×. Thus, the laser average power must also increase by around 2× or perhaps even more to maintain throughput.
It is also possible that improved performance in the optics, e.g., with the application of optical coatings and/or angle of incidence changes could be used to increase the overall laser system output, although studies by applicants' employer have indicated that there is not enough margin there to get to the required overall output energy levels and/or certain optics could not safely be modified in the effort. Thus this is not currently an available option because as currently perceived a relatively small percentage change in overall output energy is obtainable by this route alone, i.e., somewhere between ten to twenty percent.
Various assumptions and constraints may be applicable regarding illuminator component lifetimes and capabilities and the like which result in the conclusion that for a 6 kHz×33 mJ solution, pulse duration must increase by a factor of 4, and for a 12 kHz×17 mJ solution, pulse duration must increase by a factor of 2. Also, since the same degree of high polarization will be required from the laser light source, one can not use polarization coupling to combine separate laser beams to form a laser system output light pulse beam input to the scanner (though polarization coupling may be used elsewhere) and a 2× increase in power density through various components will cause increased depolarization. Whether or not the scanner (e.g., in the illuminator) can accommodate a change in laser beam size is also an issue. Some scanners may also not be able to accommodate a laser light source in the form of two input beams, e.g., as side-by-side laser beams with, e.g., interleaved pulsing. Applicants assume that laser spectral requirements will remain the same as for the equivalent lens used in single exposure systems.
One possible solution to the requirements noted above, a single XLA running at 6 kHz and with a 20-30 mJ pulse energy output from the power amplifier, has a number of problems in the effective implementation, not the least of which is the difficulties in getting to 6 kHz operation in an excimer seed to excimer amplifier gain medium multi-chamber laser system arrangement, for which applicants' assignee has proposed certain design approaches discussed in one or more co-pending applications noted above. In addition, the most likely deterrent to such an approach is unacceptably high energy density on certain critical optical elements in the XLA system at 20-30 mJ output pulse energy. Alternatively one could try to implement a single MOPA XLA operating at 12 kHz with a 17 mJ output pulse energy from the power amplifier, however, getting to 12 kHz poses a number of problems, e.g., an approximately 8× increase in chamber blower power, significantly accentuated chamber acoustic impacts on output pulse parameters, and difficulties in maintaining chamber robustness at high voltage with such a high repetition rate. Similarly, a single MOPO XLA (with a power oscillator in place of the power amplifier) operating at 12 kHz with a 17 mJ output pulse energy from the amplifier would face much the same detrimental impacts to effective operation. A Single MOPA XLA tic-toc (e.g., a master oscillator—single aperture—seeding a plurality of amplifiers—multiple apertures—and recombined back to a single output—single aperture) with excimer seed operating the MO at 12 kHz and each amplifier gain medium operating at 6 kHz with each having 17 mJ output pulse energy would suffer from the same problems, however, only in the MO. A single MOPA XLA tic-toc with solid state seed operating at 12 kHz (tic-toc to 2 multi-pass PA's at 6 kHz each, 17 mJ output pulse energy from each) is a possibility, however, this would require a high average power solid state seed laser, e.g., with about a 12 W average power output, which is not currently available. Two “standard” six kHz XLAs could be used side by side to tic-toc a total of a 12 kHz of 17 mJ output pulse energy laser pulses, if acceptable from a cost standpoint for very high power (around 200 W) lithography laser light sources, e.g., for immersion lithography. Cost of consumables may be acceptable, e.g., for each individual laser system, but the overall cost of operation of the entire system essentially doubles. Other problems need also be addressed, however the above noted are what applicants currently believe to be the “show stoppers” to the various noted configurations meeting the requirements for performance and cost of operation for very high power laser operations, e.g., for immersion lithography laser light sources.
Applicants' employer's competitor GigaPhoton has utilized multi-chamber seed laser/amplifier laser systems in a master oscillator power oscillator configuration, as shown, e.g., in U.S. Pat. No. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued on Apr. 13, 2004 to Nakao et al; U.S. Pat. No. 6,741,627, entitled PHOTOLITHOGRAPHIC MOLECULAR FLUORINE LASER SYSTEM, issued on May 25, 2004 to Kitatochi et al, and U.S. Pat. No. 6,839,373, entitled ULTRA-NARROWBAND FLUORINE LASER APPARATUS, issued on Jan. 4, 2005 to Takehisha et al. However, not without certain problems not faced by a power amplifier (i.e., a fixed amplification path—one or more passes—through the amplification medium as opposed to laser oscillation). These may include, e.g., two critical challenges in the application of the injection locking method, e.g., to lithography. They are related to ASE and coherence.
Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) refers to a multipass power amplifier. U.S. Pat. Nos. 6,816,520, entitled SOLID STATE SYSTEM AND METHOD FOR GENERATING ULTRAVIOLET LIGHT, issued on Nov. 9, 2004 to Tolloch et al., relates to mixing schemes for 193 nm light generation with a solid state seed to an excimer laser; U.S. Pat. Nos. 6,373,869, entitled SYSTEM AND METHOD FOR GENERATING COHERENT RADIATION AT VACUUM ULTRAVIOLET WAVELENGTHS USING EFFICIENT FOUR WAVE MIXING, issued to Jacob on Apr. 16, 2002, relates to mixing schemes for 193 nm light generation. US Published Patent Application, Pub. No. US2005/0185683A1 relates to frequency shifting to get 193 nm light. U.S. Pat. No. 5,233,460, entitled METHOD AND MEANS FOR REDUCING SPECKLE IN COHERENT LASER PULSES, issued to Partlo et al. on Aug. 3, 1993 discusses misaligned optical delay paths for coherence busting on the output of gas discharge laser systems such as excimer laser systems. U.S. Pat. No. 6,191,887, entitled LASER ILLUMINATION WITH SPECKLE REDUCTION, issued to Michaloski et al. on Feb. 20, 2001, relates to coherence busting for speckle reduction in a multiple delay path pulse stretcher. U.S. Pat. No. 5,940,418, entitled SOLID-STATE LASER SYSTEM FOR ULTRA-VIOLET MICRO-LITHOGRAPHY, issued to Shields on Aug. 17, 1999 relates to MOPO/PA configurations where a solid sate laser is the MO for a solid state laser PO or PA but refers to an article as describing the production of 193 nm light using an excimer laser, a dye laser and a birefringent BBO crystal for frequency multiplication harmonic generation, Muckenheim et al., “Attaining the wavelength Range 189-197 by frequency mixing in B—BaB2O4,” Appl. Phys. B 45 (1988), pp. 259-261. U.S. Pat. No. 6,031,854, entitled DIODE PUMPED CASCADE LASER FOR DEEP UV GENERATION, issued to Ming on Feb. 29, 2000 relates to a solid state cascade laser in which the output of a diode pumped solid state laser is used to pump another solid state laser to produce DUV light; U.S. Pat. No. 6,320,886, entitled LASER DEVICE, issued to Dawber on Nov. 20, 2001 relates to a solid state optical parametric generator (“OPG”) that is pumped by light produced by a pump source 4 that is disclosed also to be a solid state laser, and where the OPG is in a resonance cavity. U.S. Pat. No. 6,477,188, entitled LIGHT SOURCE, issued to Takaoka on Nov. 5, 2002, relates to solid state lasers seeding and/or pumping other solid state lasers or OPGs or OPOs. U.S. Pat. No. 6,590,698, entitled ULTRAVIOLET LASER APPARATUS AND EXPOSURE APPARATUS USING SAME, issued to Ohtsuki on Jul. 8, 2003, relates to a solid state feed of a seed into distributed fiber-optic amplifiers. U.S. Pat. No. 6,654,163, entitled OPTICAL AMPLIFIER ARRANGEMENT FOR SOLID STATE LASER, issued to Du on Nov. 25, 2003, relates to an amplifier gain medium that can be a gas discharge or solid state laser seeded from an undisclosed type of laser. U.S. Pat. No. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued to Nakao et al. on Apr. 13, 2004 discloses an F2 gas discharge laser in a MOPA or MOPO configuration with a gas discharge master oscillator seeding a gas discharge amplifier. U.S. Pat. No. 4,982,406, entitled SELF INJECTION-LOCKING LASER TECHNIQUE, issued to Facklam on Jan. 1, 1999, relates to a laser system that has so-called “self-injection locking” and appears to disclose a number of prior art systems, that inject a seed beam into an amplifier laser. U.S. Pat. No. 4,019,157, entitled METHOD AND APPARATUS FOR TUNING HIGH POWER LASERS, issued to Hutchinson on Apr. 19, 1977, relates to a pulsed gas laser (CO2) seeded with a CW laser beam from a seed laser disclosed to be a CW CO2 laser. U.S. Pat. No. 4,227,159, entitled COMMON-RESONATOR PRE-LOCKED LASER, issued to Barrett on Oct. 10, 1980 relates to a dye laser simultaneously pumped in a resonator cavity by an argon ion laser and a solid state Nd:YAG frequency doubled laser. U.S. Pat. No. 4,019,157, entitled METHOD AND APPARATUS FOR TUNING HIGH POWER LASERS, issued to Hutchinson on Apr. 19, 1977, relates to high power gas lasers which are seeded by a beam from a low power laser. U.S. Pat. No. 4,264,870, entitled AUTOMATIC LOCKING SYSTEM FOR AN INJECTION LOCKED LASER, issued to Avicola on Apr. 28, 1981, relates to an injection locked oscillator which is an optically pumped dye laser that is provided with a seed laser pulse from a “master oscillator” but this MO actually acts to create a population inversion in the ILO cavity at a wavelength selected by the wavelength of the master oscillator pulse prior to stimulated emission lasing in the ILO resulting from the pumping of the ILO flash lamp. U.S. Pat. No. 4,490,823, entitled INJECTION-LOCKED UNSTABLE LASER, issued to Komine on Dec. 25, 1984, relates to a laser system that has an optical switch to form the cavity first to include line narrowing in a stable resonator and thereafter to switch to an unstable resonator with the line narrowing package not in the cavity any longer. U.S. Pat. No. 4,606,034, entitled ENHANCED LASER POWER OUTPUT, issued to Eden et al. on Aug. 12, 1986, relates to population inversion created by a “seed” pulse before stimulated emission is caused in the amplifier by the amplifier being pumped. U.S. Pat. No. 4,689,794, entitled INJECTION-LOCKING A XENON CHLORIDE LASER AT 308.4 NM, issued to Brosman on Aug. 25, 1987, relates to an injection locked excimer gas discharge laser system, e.g., a XeCl laser which either uses line narrowing or an injection of a low level amount of radiation into the cavity to essentially do preionization so the gain achieved by the main pumping need not be so high.
Partlo et al, Diffuser speckle model: application to multiple moving diffusers, Appl. Opt. 32, 3009-3014 (1993), discusses speckle reduction techniques.
Ti:sapphire (Titanium-sapphire) lasers emit near-infrared light, tunable in the range from 650 to 1100 nanometers. These lasers are tunable and can generate ultrashort pulses. Titanium-sapphire refers to the lasing medium, a crystal of sapphire (Al2O3) that is doped with titanium ions. A Ti:sapphire laser is usually pumped with another laser with a wavelength of 514 to 532 nm, for which argon lasers (514.5 nm) and frequency-doubled Nd:YAG, Nd:YLF, and Nd:YVO lasers (527-532 nm) may be used as discussed at http://en.wikipedia.org/wiki/Ti-sapphire_laser
Second harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process, in which photons, e.g., at a given wavelength, interacting with a nonlinear material are effectively “combined” to form new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons. Only under special circumstances, the rate of conversion of photons to the higher-energy photons is significant. The two fundamental requirements for efficient nonlinear power conversion are that the pump intensity is high over a certain propagation length, and that the involved beams preserve a certain phase relationship over that length. Under properly optimized conditions, it is possible to obtain more than 50% conversion efficiency (sometimes even more than 80%) by focussing an intense laser beam into a suitable nonlinear crystal. This is widely used, for example to generate green light at 532 nm from the near infrared output of a Nd:YAG laser at 1064 nm. Some common materials used for second harmonic generation are potassium titanyl phosphate (KTP), lithium triborate (LBO), cesium lithium borate (CLBO), lithium tantalate, and lithium niobate.
As mentioned above, a high conversion efficiency requires that the input light and the second harmonic light are kept in phase. This is not the case without special measures, because the speed of light in a material generally varies with wavelength due to dispersion of the index of refraction. In some nonlinear crystals, a particular combination of crystal orientation and crystal temperature can be found where, due to birefringence, the fundamental and second harmonic light both see the same index of refraction, and so remain in phase as they propagate. In other nonlinear materials, where this is not possible, periodic poling is used to keep the waves approximately in phase. This technique, called quasi-phase matching, is commonly used for lithium niobate and lithium tantalate, and greatly expands the options for efficient frequency doubling at various wavelengths and temperatures. http://en.wikipedia.org/wiki/Second_harmonic_generation.
Acousto-optic (“AO”) crystals are often used in optical systems to modulate, frequency shift, or diffract a laser beam. In the case of frequency shifting, the beam interacts with an acoustic wave that moves inside the crystal, Bragg-reflecting from the wave. The frequency of the reflected beam is the sum of the frequency of the original beam and the frequency of the acoustic wave. Depending on its direction of motion, the acoustic wave can contribute either a positive or negative frequency component. In an AO phase shifter, the beam to be phase shifted reflects first from one AO crystal that adds a radio-frequency (RF) component, then from a second AO crystal that subtracts an RF component of the same magnitude, restoring the beam's original frequency. The phase delays between the two RF signals can be varied, adding a controllable phase shift to the beam, as is discussed at http://lfw.pennnet.com/Articles/Article_Display.cfm?Section=ARTCL&ARTICLE_ID=221417&VERSION_NUM=3&p=12
Published International Application WO 97/08792, published on Mar. 6, 1997 discloses an amplifier with an intracavity optical system that has an optical path that passes each pass of a sixteen pass path through the same intersection point at which is directed a pumping source to amplify the light passing through the intersection point.
R. Paschotta, Regenerative Amplifiers, found at http://www.rp-photonics.com/regenerative_amplifiers.html (2006) discusses the fact that a regenerative amplifier, may be considered to be an optical amplifier with a laser cavity in which pulses do a certain number of round trips, e.g., in order to achieve strong amplification of short optical pulses. Multiple passes through the gain medium, e.g., a solid state or gaseous lasing medium may be achieved, e.g., by placing the gain medium in an optical cavity, together with an optical switch, e.g., an electro-optic modulator and/or a polarizer. The gain medium may be pumped for some time, so that it accumulates some energy, after which an initial pulse may be injected into the cavity through a port which is opened for a short time (shorter than the round-trip time), e.g., with the electro-optic (or sometimes acousto-optic) switch. Thereafter the pulse can undergo many (possibly hundreds) of cavity round trips, being amplified to a high energy level, often referred to as oscillation. The electro-optic switch can then be used again to release the pulse from the cavity.
Alternatively, the number of oscillations may be determined by using a partially reflective output coupler that reflects some portion, e.g., around 10%-20% of the light generated in the cavity back into the cavity until the amount of light generated by stimulated emission in the lasing medium is such that a useful pulse of energy passes through the output coupler during each respective initiation and maintenance of an excited medium, e.g., in a pulsed laser system.
Uppal et al, Performance of a general asymmetric Nd: glass ring laser, Applied Optics, Vol. 25, No. 1 (January 1986) discusses an Nd: glass ring laser. Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) discloses a seed laser/power amplifier system with multiple passes through a gain medium in a ring configuration, which Fork et al. indicates can be “converted into a closed regenerative multi pass amplifier by small reorientations of two of the four mirrors that compose the resonator [and providing] additional means . . . for introducing and extracting the pulse from the closed regenerator”. This reference refers to an open-ended amplifier portion with fixed number of passes through the amplifier portion (fixed by the optics and, e.g., how long it takes for the beam to walk off of the lens and exit the amplifier portion) as a “resonator”.
Mitsubishi published Japanese Patent Application Ser. No. JP11-025890, filed on Feb. 3, 1999, published on Aug. 11, 2000, Publication No. 2000223408, entitled SEMICONDUCTOR MANUFACTURING DEVICE, AND MANUFACTURING OF SEMICONDUCTOR DEVICE, disclosed a solid state seed laser and an injection locked power amplifier with a phase delay homogenizer, e.g., a grism or grism-like optic, between the master oscillator and amplifier. United States Published application 20060171439, published on Aug. 3, 2006, entitled MASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, a divisional of an earlier published application 20040202220, discloses as master oscillator/power amplifier laser system with an optical delay path intermediate the master oscillator and power amplifier which creates extended pulses from the input pulses with overlapping daughter pulses.
As used herein the term resonator and other related terms, e.g., cavity, oscillation, output coupler are used to refer, specifically to either a master oscillator or amplifier portion, a power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, e.g., the size of the cavity and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, e.g., a one pass, two pass, etc. power amplifier, or six or so times in the embodiment disclosed in Fork, et al. Amplification of femtosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) and not on the operation of some optical switch in the cavity. In some of the literature an oscillator in which the round trip through the amplification gain medium, e.g., around a loop in a bow-tie or racetrack loop, is not an integer number of wavelengths, may be referred to as an amplifier, e.g., a power amplifier, while also constituting an oscillator laser. The term power amplification stage and more specifically ring power amplification stage is intended herein to cover both of these versions of a power oscillator, i.e., whether the path through the gain medium is an integer multiple of the laser system nominal center wavelength or not and whether the literature, or some of it, would refer to such an “oscillator” as a power amplifier or not. The closed loop path or oscillation loop as used herein refers to the path through the amplification gain medium, e.g., an excimer or similar gas discharge laser amplification stage, around which the seed laser pulse light oscillates in the amplification stage.
Yb3+ fiber lasers are inherently tunable, as discussed in J Nilsson et al “High-power wavelength-tunable cladding-pumped rare-earth-doped silica fiber lasers,” Opt. Fiber Technol. 10, pp 5-30 (2004).